This invention relates to methods and apparatus for degassing and filtering molten metals such as copper, particularly for use in a continuous casting process.
It is important in some metal casting processes to remove gaseous and particulate impurities from the molten metal to be cast. Copper wire, for example, is typically made by drawing down copper rod that has been rolled from a continuously cast copper bar. Gas that is not removed from the copper prior to casting may result in voids that increase the difficulty of satisfactorily drawing the wire and/or that lead to more wire breakage. Particulate impurities that are not removed prior to casting may also increase the difficulty of drawing the wire, decrease conductivity and ductility, and increase wire breakage.
Various molten metal degassing and filtering techniques are known in the art, but each of the known techniques is believed to have certain shortcomings. For example, some of the known techniques involve introducing flushing gas into the molten metal through apertured pipes. Bubbles of the flushing gas are intended to join and/or react with gas already present in the molten metal and thereby carry the pre-existing gas out of the metal. However, for a given quantity of flushing gas, the use of apertured pipes tends to produce relatively small numbers of relatively large flushing gas bubbles. It would be preferable, for more complete dispersion of the flushing gas throughout the molten metal, as well as for better utilization of the flushing gas, to have larger numbers of smaller bubbles.
As another example of shortcomings of the known molten metal degassing and filtering techniques, it is known to apply varying degrees of vacuum to the molten metal to suction or vacuum gas impurities out of the metal. The known techniques for applying relatively high vacuum necessitate expensive and complicated vessels for holding the metal being vacuumed so that excessive ambient air does not inadvertently leak into the vessel. High vacuum has also tended to be associated with batch processing of molten metal. It is difficult to subject a continuous flow of molten metal to a high vacuum because the vacuum tends to work against the molten metal flow. Low vacuum, on the other hand, tends to be of less benefit. It may also be desirable to protect the free surface of the molten metal with a gaseous atmosphere having a particular composition. The difficulty of doing this can be increased if the protective atmosphere must be maintained at a pressure below ambient atmospheric pressure (e.g., as part of a system for subjecting the molten metal to vacuum conditions as mentioned above). Any leakage into the system contaminates the protective atmosphere. And if the entire protective atmosphere must be maintained at a vacuum, the vacuum system may have to handle a relatively large volume of gas, which increases the expense of providing and operating the vacuum.
In view of the foregoing, it is an object of this invention to provide improved and simplified molten metal degassing and filtering methods and apparatus.
It is a more particular object of this invention to provide molten metal degassing methods and apparatus which are capable of dispersing flushing gas more thoroughly in the molten metal.
It is still another object of this invention to provide molten metal degassing methods and apparatus which allow both a protective atmosphere and relatively high partial vacuum to be employed with greater economy and efficiency.
It is yet another object of this invention to provide molten metal degassing and filtering apparatus which can be readily applied to a continuous flow of molten metal, as is generally desirable for a continuous costing operation.